GANA's Glass Fab Opens with Presentations on Making and Breaking Glass

The Glass Association of North America's (GANA) Glass Fabrication Educational Event kicked off this morning in Kansas City, Mo., with a presentation on Float Glass Technology by Ivan Zuniga of AGC Flat Glass North America. The presentation was the perfect kick-off for those 30 attendees who came out to Kansas City a day early to tour AGC's Spring Hill, Kan., plant. Zuniga's presentation provided a thorough and informative background to the tour.

Today Zuniga walked his listeners through the manufacturing process from charging the raw materials to melting in the furnace, floating along the tin bath, to annealing and, finally, cutting. Zuniga related the process to his audience's experience, explaining, "For those of you who contact us and say 'well I've got an architect and he's got a LEED project and he wants to know the number of regional materials I have for this product ...'" Zuniga said they're able to look at the source of each of the products in the glass' raw batch: silica sand, cullet, soda ash, dolomite, limestone and other products. "The other thing you guys ask about is recycled material," Zuniga continued. "Well we do use crushed glass, which is recycled. But the way LEED reads," he explains, recycled content must be from a post-consumer product. "So according to LEED and according to the ISO definition, cullet does not comply as a recycled content."

On that note, Zuniga fielded a question from a listener about how float plants manufacture low-iron glass such as was seen on-site during yesterday's tour. "How do you take out the iron?" he asked. Zuniga explained that the product is more expensive than typical float glass because taking the iron out isn't really an option; rather, you use more "exotic" materials, he explained, that have much lower deposits of iron oxide. The cost is higher because, he continued, "they definitely are more difficult to get and you have to get them from further away than you would normally." Zuniga added, "It's all about transportation of our materials. That's really one of the higher costs." The further away materials are being imported from, the more expensive it's going to get. Following Zuniga's presentation on making glass, Pilkington North America's David Duly took the podium to discuss breaking glass. Duly, too, discussed raw batch materials, noting that the addition of ingredients such as calcium and sodium are what gives glass its structure and strength. Duly went on to explain that there are very high stress concentrations in glass. For starters, Griffith Flaws are stresses invisible to the eye but inherent throughout the glass. "What these flaws do is when glass is strained the strain along the glass can be concentrated due to this Griffith Flaw and when strain exceeds the strength of the atomic bonds, the glass breaks." As Duly pointed out, these are just the flaws existing in glass, then there are the flaws added to glass, such as scratches, etc.

Duly provided his audience an overview of the breakage characteristics of glass types; annealed glass leads to large broke fragments with sharp edges; heat-strengthened glass results in a break pattern similar to annealed but, he says, with fewer pieces than with annealed; and tempered glass breaks into hundreds of small fragments. Next he went on to answer the question of what causes glass to break.

"We design glass in buildings based on the fastest mile-per-hour wind speed based on building codes," he began. Duly noted that this information is used to determine uniform pressure load, taking into consideration factors such as building height, location, the surroundings of building, etc. Another factor to consider is, of course, projectiles, with consideration of whether the object is a hard or soft body (a bullet versus a person bumping into the glass) and the velocity of the projectile, among other factors. And while, he said, "we don't see it much anymore," Duly noted that onsite construction activities such as grinding and welding are known to lead to breakage, as well.

A bigger concern today, Duly said, is thermal stress. He noted that the prominence of energy-efficient products absorbing heat can lead to thermal stress issues. Duly explained that a classic thermal stress fracture always begins at the glass edge, running from a 90 degree angle before meandering away from the edge of glass in any direction to escape the stress. He added, "When you see a single fracture, in most cases the edge is damaged and the stress at the fracture was low." More fractures could mean a good edge simply couldn't handle the extreme temperature difference between the center and edge of glass.

Duly ended his presentation by asking what we can do to improve the strength of glass. "The most common way to improve the strength is to heat treat glass," he said. "When you heat treat glass you're putting the outer surface in compression and the inner surface under tension." The compressive forces counter the opening of crack tips, he explained. The next step, he said, is "obviously if you take more care in handling the glass, you can prevent some of the flaws on the surface from opening up."